This invention relates to magnetometers, and more particularly to fiber optic magnetometers.
The possibility of detecting weak magnetic fields by using magnetostrictive perturbations has been discussed in "Detection of Magnetic Fields Through Magnetostrictive Perturbations of Optical Fibers", Optics Letters, Vol. 5, No. 3, March 1980. The basic principle of operation of such a sensor involves the measurement of longitudinal strain induced in an optical fiber by a dimension-changing magnetostrictive element. When the magnetostrictive material is in the presence of a magnetic field it is caused to undergo dimensional changes which stretch the optical fiber so as to lengthen its optical path. This causes a detectable phase shift in light passing therethrough when compared to light passing through an optical fiber of a reference arm which is not affected by the magnetic field. Reference may be made to an article entitled "Optical Fibre Magnetic Field Sensors" by Dandridge et al, Electron. Lett., Vol. 16, pp. 408-409 (1980), and "Characteristics of Fiber-Optic Magnetic-Field Sensors Employing Metallic Glasses"by Koo et al., Optics Letters, Vol. 7, No. 7, pp. 334-336 (July 1980) for disclosure of prior art magnetometers.
The sensitivity of fiber optic magnetometers is determined by the long lengths of active sensing element (termed the sensing arm) and the ability to measure small signal induced phase shifts. In the fiber optic sensors built to date, the signal band of interest has typically been above 10 Hz. In this range the minimum detectable phase shift is about 10.sup.-6 rad. At frequencies below 10 Hz, the performance of the magnetometer is degraded by the following factors;
(i) intrusion into the signal band of the intensity noise (IN) of the laser. (ii) intrusion into the signal band of the phase noise (PN) of the laser, (iii) spurious signals due to differential thermal drift (TD between the arms). The frequency spectrum of i and ii has a 1/f frequency dependence (yielding a typical 10.sup.4 degradation in sensitivity at 0.1 Hz), and (iii) manifests itself at very low frequencies close to and at DC, making DC measurements almost impossible. Effects ii and iii may also cause signal fading in the device, as well as a nonlinear response and frequency up conversion.